Melt and high-density fluid inclusions of magmatic water in phenocrysts of quartz from acidic volcanic rocks of the Verkhneural’sk ore district, South Urals

Abstract

The location of the vast majority of the Ural VHMS deposits in central volcanic structures and their paragenetic relation to acidic volcanism are the most reliable attributes. According to the genetic models, the formation of the VHMS systems is related to shallow chambers of acidic magma. However, many aspects of such relation are discussed. On the basis of comprehensive study of melt inclusions in quartz phenocrysts in acidic volcanic rocks, we determined the composition of primary magmatic melts; the contents of trace elements, H 2 O, Cl, S; and the pressure and composition of magmatic fluid for the first time in the Verkhneural’sk ore district, South Urals. The Verkhneural’sk ore district located in the northern Magnitogorsk trough is attributed to the Uchaly‐Alexandrinka structural‐formation zone. Eight Cu‐Zn deposits are located in the district. The geology of the district, which has been described in numerous publications [1‐3], permits consideration of only some features that are important for our study. Acidic volcanic rocks occur in the upper part of the bimodal basalt‐rhyolite association of the Middle Devonian Karamalytash Formation. According to pertochemistry, the underlain basalts are tholeiitic, while acidic volcanics mainly pertain to calc-alkaline rock series. All economic deposits of the district are hosted in acidic volcanic rocks at the three lithostratigraphic levels corresponding to three rhythms of volcanism. Melt inclusions were studied in 150 samples of dacite, dacitic rhyolite, and rhyolite from these three rhythms. The inclusions range from 5‐10 to 100 µ m in size. Variably devitrified inclusions are the most abundant; vitreous inclusions locally with a gas phase are less frequent. Melt inclusions containing opaque crystals determined as magnetite and globules of pyrrhotite, pentlandite, chalcopyrite, and bornite are also observed. Glasses were soften at 600‐720 ° C and homogenized at 850‐1130 ° C. Initial melting of devitrified inclusions was observable at 780‐850 ° C, and total homogenization, at 950‐1210 ° C [4].